Modulator arrangement for carrier wave telegraphy and telephony



June 22, 1948. K. R. VALE 2,443,754

MODULATOR ARRANGEMENTS FOR CARRIER WAVE TELEGRAPH! AND TELEPHONY Filed Dec. 14, 1945 AM 130 07 g INVENTOR Kama fizf.

ATTORNEY Patented June 22, 1948 MODULATOR ARRANGEMENT FOR CARRIER VAVE TELEGRAPHY AND TELEPHONY Keith R. Vale, Sydney, New South Wales, Australia, assi'gnor to Amalgamated Wireless (Australasia) Limited, Sydney, New South Wales, Australia,'a company of New South Wales,

Australia Application December 14, 1945, Serial No. 635,098 In Australia March 6, 1945 6 Claims. 1

This invention rel-ates to improvements in modulation arrangements for carrier wave telegraphy and te'lephony and is directed to the production of an improved modulator circuit which permits both carrier and modulating voltages to be suppressed in the output circuit.

In carrier wave telegraphy and telephony, and in privacy devices for telephony and in similar application, it is often necessary to modulate a carrier and to provide means for suppressing the transmission of each of the input waves and for producing in the output of the modulator only waves which represent. the combination frequencies and the double frequency of the impressed waves.

In the past, in order to perform this function, it has been the practice to employ various arrangements of the well known balanced modulator' circuit. All of these prior art modulator circuit arrangements require the use of balanced input and/or output transformers and their eflicient' operation depends on securing and maintaining a high degree of balance; a requirement that is not readily realizable in practice.

The object of the present invention is to provide a comparatively simple circuit utilizing two valves interconnected in such a way that both carrier and modulation can be suppressed, said circuit having the advantage that the use of balanced transformers is avoided and all transmission pathsare earthed on one side, i. e., single-side. A further advantage arising out of performance of the present invention is that exact independent balance can be obtained by simple means for both carrier and modulation suppression.

The above objectives are achieved in accordance with the present invention by an improved modulator arrangement comprising a pair of similar thermionic valves each having at least a cathode, an anode and a control grid, an unbalanced input circuit for carrier frequency energy, and an' unbalanced input circuit for modulation frequency energy, both said circuits being connected to said valves in such a manner as to produce, without the aid of a balancing transformer; instantaneous potentials of opposite phase relationship on the control grids of'both valves, andmeans including an unbalanced output circuit for obtaining energy at a frequency equal to the sum and/or difference frequencies of the said carrier and modulating frequency energies.

Incarrying out the invention one of said thermionic valves is provided with an unbypassed resistive impedance in its cathode circuit and has V2 is identical to VI.

its control grid effectively earthed for all operating frequencies. Carrier frequency energy and modulating frequency energy from the unbalanced input circuits are applied simutaneously to the cathode of this valve and to the grid of the other valve. The resistive impedance in the cathode circuit referred to functions to invert the phase of the energy applied to its associated cathode and thus produce on the control grid of this valve input potentials of opposite phase relationship to those applied to the control grid of the other valve.

The anodes of the valves are connected'together and to an unbalanced output circuit which is appropriately tuned to receive either the sum and/or diiference frequencies of the said carrier and modulation frequency energy as desired.

For a more complete understanding of the invention and the manner in which it is to be carried out, attention is now directed to the following description in connection with the accompanying drawing, which illustrates one practical method of carrying the invention into effect.

Referring to the drawing, valve VI is of the pentode type having its cathode I earthed' through resistor l and condenser 9 in parallel.

Control gridB is connected through resistor H to a source of fixed bias represented by battery I2 Screen grid 5 is bypassed for all signal frequencies by con-denser 8 connected to cathode I. Operat-' ing voltage is supplied to the screen 5 from apotentiometer 34, 35 across a source of high tenslon voltage HT2 through resistor 29. The suppressor grid 4 is connected to cathode 1. Valve Cathode resistor 22 is not bypassed. Control grid 26 is effectively earthed for all frequencies through condenser 19 and is connected through resistor 20' to a variable source of bias potential indicated by 2|. Screen grid is effectively shorted to cathode, at all frequencies through condenser 28 and is supplied with high tension operating voltage through resistor 30 and potentiometer 34, 35. Plates 3 and 23 are coupled together and to a tuned circuit.

3|, 32 which is resonant at the carrier frequency and plate operating voltage from source HT! is supplied through coil 32. Modulating potentials are applied to the terminals A. F. input of low frequency transformer l5. Similarly, carrier fre-, quency voltage is applied to terminals R. F. in-

put of transformer Hi. The secondaries of these transformers, in series with resistors l4 and I1 respectively, are connected in parallel. Audio frequency voltages developed across resistor M are fed through condenser l3 to grid 6 of valve VI and, through condenser l'8, to cathode 21 of valve V2. Similarly, radio frequency voltages across I! are also fed to grid 6 and to cathode 21. Note that the secondary of transformer i5 is effectively shorted for radio frequencies by self-capacity as represented by condenser 36.

Valves Vi and V2 should be of the type having a square-law characteristic, i. e., the relation between instantaneous plate current (i) and instantaneous grid-cathode voltage (e) can be expressed for valve V! by a relation over a substantial portion of the characteristic. ype 6U7G valves have been found suitable.

It is preferable to use a valve having a screen grid so that inverse feedback effects, arising in the unbypassed cathode resistor 22, are substantially compensated by variations of screen voltage obtained by connecting screen to cathode.

In a particular circuit the following values were used:

The two high tension sources, which may be the same source, were 250 v. D. C. Bias it was 6 volts, screen grid voltage 100 volts by making 34==32, 000 ohms, 35=25,000 ohms. The tap on bias source H is adjusted so that the bias on the control grid of 25 is equal to that of valve Vi. Note that in each valve the bias is the sum of a small self-bias developed across the bias resistor in the cathode circuit, and a fixed or adjustable bias. Since resistor 22 is not bypassed, and if the value of this resistance is considerably greater than the reactance of condenser 58, the voltage applied betweengrid and cathode of valve V2 is equal in value but opposite in sign to that applied to the grid-cathode of valve Vi. This is true since grid 26 is effectively earthed through condenser l9.

Thus, if each valve is operating on an identical characteristic of square-law shape, the plate current of valve 2 is represented:

e=El cos wZ-l-E'Z cos pt (Phases are neglected to simplify the illustration.) The total plate current is therefore: i =2a +2a (El cos wt+E2 cos pt) E1E2 cos (w-l-P) .+ElE2 cos (or-In Thus the output of the device contains terms representing steady direct currents, alternating currents twice the frequency of the signal and carrier respectively, and two radio frequency currents of the sum frequency and difierence frequency respectively. It is seen that 'there are no terms present for the frequencies to or p. Thus, if the tuned circuit 3|, 32 resonates at frequency w and is reasonably selective to reject frequencies 2 and 2p, the two sidebands only may be obtained by coupling to coil 33 as shown. If the two frequencies are nearly equal, then the output circuit may be tuned to reject all except one sideband. If voltages of frequency a: do appear at the output, then a slight adjustment of bias 2| will be sufficient to ensure that the valves are operating on identical characteristics. In effect, this adjustment is one which makes the constants on equal in the two expressions for the characteristics.

Although for purposes of description the signal frequency has been shown as an audio frequency, it may be of any frequency.

What is claimed is:

1. In a current mixing system, two electron discharge devices each having electrodes including an anode, a cathode, and a control grid, two sources of currents ,to be intermodulated, an impedance between the, cathode of one device and ground, means coupling said sources in parallel in shunt to said impedance, means coupling said sources in parallel between the grid and cathode of the other device, and an output circuit coupling the anodes of. said devices in parallel.

2. A system as recited in claim 1 wherein said one device has a screen electrode coupled to its cathode by a capacitor of low impedance to voltages of the frequencies of said sources.

3. In a modulation system, a source of signals, a source of wave energy to be modulated, a pair of electron discharge devices, an unbalanced input circuit coupled to an electrode of one device, an unbalanced input circuit coupled to an electrode of the other device, an impeder in each input circuit, a coupling connecting said impeders in parallel, connections from the signal source to one impeder for applying signals to both input circuits, connections from the Wave energy source to the other impeder for applying carrier wave energy to be modulated to said input circuits the arrangement being such that the signals and wave energy are applied in phase opposition to the devices and an output circuit coupling the anodes of said devices in parallel.

4. A system as recited in claim 3 including means to tune said output circuit to a side band frequency. I

5. In a modulation system, a source of signals, a source of wave energy to be modulated, a pair of similar electron discharge devices each having an anode, a cathode and. a control grid, an unbalanced input circuit including an impede-r coupled between the control grid of one of said devices and ground, an unbalanced input circuit including an impeder coupled between the oathode of said other device and ground, a coupling connecting said impeders in parallel, connections from said signal source to one impeder for applying signals to said, one impeder, connections from said wave energy source to the other impeder for applying carrier wave energy to be modulated to the other impeder and an output circuit cou-- pling the anodes of said devices in parallel.

6. In a modulation system, a source of signals,

a source of wave energy to be modulated, a pairof similar electron discharge devices each having,

an anode, a cathode and a control grid, an unbalanced input circuit including an impeder coupled between the control grid of one of said devices and ground, an unbalanced input circuit including an impeder coupled between the cathode of said other device and ground, a coupling connecting said impeders in parallel, connections from the signal source to one impeder for applying signals to said one impeder, connections from the wave energy source to the other impeder for applying wave energy to be modulated to the other impeder, a third impeder coupling the cathode of the other device to ground, a screen grid in said other device, a coupling of low impedance to voltages of signal and wave energy frequency coupling the screen grid of the other device to ground, and an output circuit coupling the anodes of said devices in parallel.

KEITH R. VALE.

REFERENCES CITED The following references are of record in the file of this patent: 

